This invention relates to propellants and particularly to "gun propellants" which are generically defined herein as particulate solid propellants for propelling projectiles. Specifically, this invention is concerned with inhibitors for gun propellants in which the inhibitor is the reaction product of a polyfunctional isocyanate and a polyol. Suitable propellants are those containing an energetic or non-energetic cellulose binder.
At the outset, a clear distinction should be drawn between "solid propellants," as used in rocket engines, and "gun propellants," which are used to propel the projectiles of pistols, rifles, artillery pieces, and other types of guns. One major distinction can be found in configuration. The term "solid propellant," as used in rocket engines, refers to a single, cohesive grain which fills and is bonded to the case of the rocket engine. These solid propellant grains may be monolithic, in which instance they are intended as "end burning" grains; or may be generally cylindrical and formed with a single opening extending axially therethrough which may be star-shaped or otherwise configured in cross-section, in which instance they are intended as "internal burning" grains. In either instance, the solid propellants will have diameters of several inches to several feet, and lengths ranging from one foot to greater than 50 feet. In contrast, the term "gun propellant" refers to a plurality of particulate grains which are loosely contained within a metal case or cloth bag. For small arms, the individual grains of a gun propellant are cylindrical, but have diameters of only a few hundredths of an inch. For larger guns, such as artillery pieces, the individual grains may have diameters up to about one-half inch and lengths up to about 2 inches.
Another significant distinction between solid propellants and gun propellants is found in their reaction times. Solid propellants are intended to burn only on a single surface at substantially uniform rates for intervals of several seconds to several minutes at uniform pressures of the order of 1000 psi. In contrast, gun propellants are intended to burn completely in less time than is required for the bullet or other projectile to reach the end of the gun barrel, usually only a few milliseconds. In reality, the gun propellants provide a substantially instantaneous explosion, creating pressures of the order of 50,000 to 60,000 psi, which are contained by the chamber of the gun. This requires that the chamber portion of guns be constructed heavily and massively in order to contain these explosions. With prior art gun propellants, some effort has been made to control or limit the burning rate in order to cause the energy of the propellant to be expended over the entire interval while the projectile is travelling through the barrel of the gun. By doing this, the chamber pressures are significantly reduced which permits the guns to be produced more economically and to have increased service life. Furthermore, this controlled burning of the gun propellant provides improved pressure distribution during the propulsion of the projectile which serves to improve the piezometric efficiency and thus increase the muzzle velocity provided by a given gun propellant.
For any given gun propellant system, interior ballistic theory can be utilized to define an optimum pressure-time profile and consequently a velocity-time profile. This most desirable profile is one in which relatively high velocities are attained at moderately low peak chamber pressures.
It has also been proposed heretofore to control the burning rate of gun propellants by the use of "deterrent" materials which serve to retard the burning rate of the grain material and are applied by impregnating the deterrent into the surface of the grain. However, when this is done, the depth of the impregnation cannot be accurately controlled. Consequently, the effect of the deterrent varies from grain to grain. Moreover, the deterrents serve to actually reduce the burning temperature of the gun propellant and, hence, compromise the performance.
In contrast, in accordance with the present invention, it is proposed to coat the gun propellant grains with an inhibitor material which bonds to, but does not impregnate the grain. Inhibitors are essentially inert chemicals which do not burn in the reaction time of gun propellants and which are applied only to the surfaces of the propellant grain where it is desired to prohibit burning. This causes the grains to burn progressively, but does not affect the burning characteristics of the grain. Inhibitors can be applied by bonding sheets of inhibitor material to the surfaces to be restricted, by wrapping the grain with selected tapes, by dipping the grain into the desired inhibitor, or by spraying the inhibitor coating onto the surface of the grain during extrusion, or the like.
The primary requirement for the inhibitor is that it must possess excellent bonding characteristics and adequate thermal resistance in order to survive the complete ballistic cycle. An additional requirement is that total curing occurs. If only partial curing occurs, the inhibited surface from one grain may adhere to the uninhibited surface of an adjacent grain, thereby further reducing the burning surface. Prediction of the instantaneous burning surface would not be possible under these circumstances, and the desired pressure-velocity-time curves would not be obtainable on a repetitive basis. The complexity of the problems that would occur if uncured inhibitor is present would be magnified many times over, since a very large number of propellant grains are typically packed into each cartridge.
Accurate control of the inhibitor thickness must be accomplished regardless of the method employed for deposition. The amount of tolerable performance degradation due to the volume occupied by the inert inhibitor will vary depending upon the individual grain dimensions; however, loss of no more than 1 to 2 percent in the variable energy is a desirable upper limit.